The current trend of requiring high performance, lightweight, and large battery for automobile power sources is accelerating demands on secondary batteries such as lithium ion secondary batteries, lithium ion polymer batteries and super-capacitors (including electric double-layer capacitors and pseudo-capacitors) having high energy density and high capacity.
However, conventional lithium ion secondary batteries using a polyolefin separator and a liquid electrolyte, as well as conventional lithium ion polymer batteries using a polymer electrolyte such as a gel polymer electrolyte membrane or a gel-coated polyolefin separator are very disqualified as high energy density and high capacity batteries in terms of heat resistance.
The separator, which is disposed between a cathode and an anode in a battery, functions to insulate these electrodes from each other, retains an electrolyte solution to provide a conduit for ionic conduction, and has a shutdown function, i.e., when the temperature of the battery increases excessively, the separator partially melts to close its pores, thereby blocking an electric current. When the temperature gets higher, the separator melts, and then a large hole is created, causing a short circuit between the cathode and the anode. This temperature is called a short circuit temperature. Generally, a separator should have a low shutdown temperature and a higher short circuit temperature. Further, a polyethylene separator shrinks at a temperature of 150° C. or higher, which results in exposure of parts of electrodes, undesirably causing a short circuit.
Therefore, in order to manufacture a high energy density and high capacity secondary battery, a separator is required, which exhibits a low heat shrinkage rate due to high heat resistance and also exhibits improved cycle performance due to high ionic conductivity.
As such a separator, US Patent Publication No. 2006/0019154 A1 discloses a polyolefin separator coated with a porous heat-resistant resin, which is manufactured by impregnating a polyolefin separator (air permeability of 200 sec/100 ml or less) in a solution of polyamide, polyimide or polyamideimide having a melting point of 180° C. or higher, and then immersing the resulting separator in a liquid coagulant, thereby removing a solvent therefrom by extraction.
Japanese Patent Laid-open Publication No. 2005-209570 discloses a polyolefin separator coated with a heat-resistant resin, which is manufactured by, in order to secure sufficient stability for high energy density and large battery with large capacity, coating both surfaces of a polyolefin separator with a heat-resistant resin solution such as an aromatic polyamide, polyimide, polyethersulfone, polyetherketone, and polyetherimide having a melting point of 200° C. or higher, and then subjecting the resulting separator to immersion in a liquid coagulant, water-washing and drying in order. Also, in this case, in order to reduce the deterioration of ionic conductivity, a phase separating agent for imparting porosity is added to the heat-resistant resin solution and the amount of coated heat-resistant resin is limited to 0.5˜6.0 g/d.
However, immersion in the heat-resistant resin or coating therewith blocks the pores of the polyolefin separator and thus restricts movement of lithium ions, undesirably deteriorating charge-discharge properties. Even when the pore structure of the polyolefin separator is not closed, a polyolefin separator which is typically used has a porosity of about 40% and a pore size of several tens nm, and thus manifests an ionic conductivity that is insufficient for large-capacity batteries.
U.S. Pat. No. 6,447,958 B1 discloses a method for preparing a heat-resistant separator by coating porous woven fabric, nonwoven fabric, paper, or a porous film made of materials such as polyolefin, rayon, vinylon, polyester, acryl, polystyrene, nylon, etc., with a liquid slurry comprising a ceramic powder and a heat-resistant nitrogen-containing aromatic polymer, and then immersing the resulting product in a liquid coagulant, thus removing the solvent therefrom by extraction. However, this method is problematic because a series of processes are considerably complicated, which leads to high manufacturing cost.
Japanese Patent Laid-open Publication Nos. 2001-222988 and 2006-59717 disclose a method for preparing a heat-resistant electrolyte membrane which is manufactured by impregnating a support such as woven fabric, nonwoven fabric, cloth or a porous film of polyaramide or polyimide having a melting point of 150° C. or higher in a gel electrolyte of a polymer such as polyethylene oxide, polypropylene oxide, polyether and polyvinylidene, or by coating the support with the polymer gel electrolyte. In this case, however, movement of ions in the support or the heat-resistant aromatic polymer layer is still restricted as in the separator or the gel electrolyte of a conventional lithium ion battery.
International Publication No. WO 05/057700 A1 discloses a porous composite film manufactured by electrospinning a polymer solution containing solid particles including fine inorganic particles. However, non-uniformity of the polymer solution containing a high concentration of solid particles disadvantageously causes clogging of nozzles used in electrospinning and non-uniform distribution of agglomerated solid particles on the surface of ultrafine fibers generated by electrospinning, which brings out formation of defects in the composite film and lowering of a mechanical property. A battery comprising the porous composite film as a separator results in remarkably decreased electrochemical stability due to electrochemical instability of the exposed solid particles, as well as shows poor heat resistance.
As mentioned above, conventional separators or electrolyte membranes do not exhibit satisfactory heat resistance and ionic conductivity at the same time, and, therefore, they are difficult to be used in manufacturing a high energy density and high capacity battery.